BEIRUT (AP) — A bomb struck a car wash Saturday in Aleppo, killing at least five people, a day after government troops opened fire to break up large protests against a violent university raid in Syria’s largest city.

Aleppo, an important economic hub, has largely stayed out of the revolt against President Bashar Assad that erupted nearly 14 months ago, but the raid on Aleppo University that killed four students earlier in the week has swelled the crowds of protesters.

On Friday, thousands marched against the university crackdown in what activists said were the largest protests in the city yet. However, it remained unclear if the regime is losing major ground in Aleppo.

Bomb attacks have become more common in Aleppo and the capital of Damascus, often targeting buildings associated with the security services, as the uprising grows increasingly militarized. However, the rebel Free Syrian Army, one of the largest armed groups, denied reports that it had claimed responsibility for Saturday’s blast.

The Britain-based Syrian Observatory for Human Rights, which relies on activists inside Syria, said five people were killed in the blast at the car wash.

Aleppo activist Mohammed Saeed said the car wash in the city’s southern Sukari neighborhood was owned by a man who serves in pro-government militias known as the shabiha. He put the death toll at six.

It was impossible to independently verify the casualty toll or other activist claims because Assad’s regime has prevented most media from working freely in the country.

Aleppo has been showing signs of increasingly unrest since Thursday’s raid of university dorms in which four students were killed and dozens arrested. On Friday, security forces trying to break up widespread protests shot and killed a 16-year-old.

The U.N. said in late March that more than 9,000 people have been killed since the uprising started in March 2011. Since then, more have been killed every day, with activists reporting daily death tolls that at times reached several dozen. On Friday, the main day for weekly anti-regime protests, at least 38 people were killed across Syria, the Observatory said.

A truce was meant to take hold April 12, as part of a peace plan for Syria brokered by special envoy Kofi Annan. The deal has helped decrease violence in some areas, and brought 40 U.N. observers to Syria, but fighting has continued.

Annan and U.N. chief Ban Ki-moon have largely blamed the regime, which continues to attack opposition strongholds and refuses to withdraw troops from the streets. However, rebels also have kept up bombing and shooting attacks on soldiers and checkpoints.

In amateur video posted Saturday, a U.N. observer in a bright blue beret and body armor inspects what residents of the town of Taftanaz tell him is a mass grave. The international group Human Rights Watch has said regime soldiers raiding the town on the Turkish border in early April killed 35 detained civilians execution-style and opened fire on others trying to flee.

The video shows a long line of headstones, placed close to each other in front of fresh mounds of earth. The observer is told each stone marks a grave. The observer walks along the stones, tapping some as he counts silently.

“Ok, I counted 52, including one soldier who refused orders and was executed as well,” the observer says. Villagers tell him five more were burned beyond recognition and did not get a headstone.

“I feel very sad about this,” the observer says, then arranges to return later to pick up a list with the names of the dead.

The video’s authenticity could not be verified.

In more violence Saturday, an explosive planted under an army vehicle in Damascus blew up, damaging nine cars.

The blast shook a downtown neighborhood near a military food cooperative, and left a crater in the street, according to an Associated Press reporter who visited the scene.

On the outskirts of the capital, tanks and troops combed through fields near the Barzeh neighborhood in the northeast and near Hamouriya in the east, activist Omar Hamzeh said via Skype.

“They are moving through the fields and firing anti-aircraft guns,” he said, adding that the regime was probably looking to arrest activists. “The rural areas around there have been very active in protests against the regime.”

The Observatory also reported army raids in Barzeh.

In the Damascus district of Kfar Souseh, dozens of protesters hurled stones at security forces after a funeral for five people killed by regime gunfire a day earlier. Amateur video from Kfar Souseh showed troops in olive-colored uniforms forming a cordon and marching toward the protesters, some of whom flashed V for victory signs. Later, burning tires sent thick, black smoke into the sky.

World powers remain divided on how to stop Syria’s crisis, though all have endorsed the Annan plan, which envisions political dialogue between Assad and his opponents once the bloodshed stops.

Annan’s spokesman Ahmad Fawzi, however, said Friday the international envoy believes his peace plan for Syrian remains “on track” — a day after the Obama administration offered a far bleaker view, saying the plan might be doomed.

U.N. peacekeeping chief Herve Ladsous said the U.N. observer team is to grow to 65 by Sunday and to 300 by the end of May.